The present invention relates to a cathode ray tube to be used for producing a color video display and, more particularly, to a shadow mask color cathode ray tube for displaying a high resolution image.
To obtain a high-resolution color cathode ray tube, it is necessary to reduce the hole pitch of a shadow mask. However, if the hole pitch of a shadow mask is reduced, the thickness of the shadow mask becomes small, so that the strength of the shadow mask in particular becomes a problem.
To cope with this problem, a technique is disclosed in Japanese Patent Laid-Open No. 172141/1985 in which the sizes of the obverse and reverse holes of each aperture of a shadow mask are made to be approximately equal to each other, thereby increasing the equivalent thickness of the shadow mask, so as to increase the mechanical strength of the shadow mask.
In addition, to prevent halation, a shadow mask has been proposed which has a peripheral portion in which the sizes of panel-side apertures are enlarged.
Japanese Patent Publication Nos. 2696/1980 and 2697/1980 describe electron beam passing holes each having a large hole which is formed in such a manner that a diameter taken in a radial direction of the shadow mask is larger than a diameter taken along a periphery of the shadow mask. In accordance with the description of each of Japanese Patent Publication Nos. 2696/1980 and 2697/1980, the mechanical strength of the shadow mask is improved in the peripheral portion. Japanese Patent Publication Nos. 2696/1980 and 2697/1980 also describe the art of deviating a small hole with respect to a large hole toward the center of the Shadow mask.
Japanese Patent Laid-Open No. 185807/1996 (U.S. Pat. No. 5,730,887) discloses that, in the peripheral portion of a shadow mask, aperture portions formed on the electron-gun side of the shadow mask have an elongate elliptical shape extending in the direction of the incident electron beams, while aperture portions formed on the screen side of the shadow mask are deviated with respect to the aperture portions formed on the electron-gun side of the shadow mask.
Japanese Patent Laid-Open No. 10335/1992 discloses that electron beam passing holes each having an elliptical aperture on the panel side of a shadow mask and a circular aperture on the electron-gun side of the shadow mask are formed in the peripheral portion of the shadow mask.
However, there has not yet been a flat shadow mask which can compatibly satisfy requirements of both strength and halation characteristics.
FIG. 11 is a cross-sectional view of an electron beam passing hole located in the central portion of a shadow mask (hereinafter referred to as the central electron beam passing hole), and relates to a known cathode ray tube. As shown in FIG. 11, in the central portion of the screen, an electron beam B is made incident on the shadow mask at approximately right angles.
FIG. 12 is a cross-sectional view of an electron beam passing hole located in the peripheral portion of a shadow mask (hereinafter referred to as the peripheral electron beam passing hole), and relates to a known cathode ray tube. In the peripheral portion of the screen, an electron beam B is made incident at an angle A on the electron beam passing hole of the shadow mask (the angle A will be hereinafter referred to simply as the electron beam angle).
In the peripheral electron beam passing hole, a central axis CH1 of the aperture of its panel-side hole (hereinafter referred to as the upper hole) and a central axis CH2 of the aperture of its electron-gun-side hole (hereinafter referred to as the lower hole) are deviated from each other. This is intended to prevent the problem of an electron beam colliding against a slant inner surface 21 of the upper hole, whereby an electron beam B1 is reflected and scattered toward the fluorescent-screen to cause the deterioration of a display image (so-called halation).
In addition, in the electron beam passing hole, the upper hole is formed to be larger than the lower hole in order to prevent an electron beam B2 reflected by a side wall of the lower hole from striking the fluorescent screen (so-called halation). Reducing the etching quantity of the lower hole has been considered to be indispensable for preventing halation. Incidentally, the term xe2x80x9cetching quantityxe2x80x9d represents the volume by which the material of the shadow mask is removed by etching.
If an acceleration is applied to the whole of this shadow mask from the electron-gun side toward the panel side, compressive stresses acting toward the center of the shadow mask are produced in the shadow mask having a radius of curvature which is convex on the panel side. In the conventional shadow mask, the panel-side etching quantity is larger than the electron-gun-side etching quantity, but since the radius of curvature is small, concave deformation can be restrained. However, the following relationship becomes conspicuous; as the radius of curvature of a shadow mask becomes larger, the mechanical strength of the panel side of the shadow mask becomes less resistant to compressive stress than that of the electron-gun side of the shadow mask. As a result, the shadow mask becomes easily deformed into a concave shape, so that it is difficult to maintain the mechanical strength of the shadow mask to a sufficient extent.
An object of the present invention is to restrain a shadow mask which is convex toward a panel side from being deformed into a concave shape when a compressive stress is applied to the shadow mask toward the center of the shadow mask, thereby realizing a mechanical strength capable of maintaining the convex shape.
To achieve the foregoing object, the present invention provides a cathode ray tube which is constructed as follows.
A shadow mask includes electron beam passing holes each having circular aperture shapes on both a panel side and an electron-gun side, and the ratio of an etching quantity on the electron beam exit side (panel side) of the central plane of the thickness of the shadow mask (hereinafter referred to simply as the upper-side etching quantity) to an etching quantity on the electron beam entrance side (electron-gun side) of the central plane of the thickness of the shadow mask (hereinafter referred to as the lower-side etching quantity) is 1.8 or less.
In addition, in each electron beam passing hole located in proximity to a imperforate area, the aperture shape of its upper hole is an elliptical shape having a longer diameter extending in a radial direction of the shadow mask, and a shorter diameter of the elliptical shape is made smaller than the diameter of the upper hole of each electron beam passing hole located in the central portion of the shadow mask.